1. Field of the Invention
The present invention relates to liquid crystal displays and, in particular, to silicon-backed microdisplays.
2. Description of the Related Art
Conventional liquid crystal displays (LCDs) include two sheets of glass arranged to form a thin cell, which is filled with a liquid crystal material. One of the sheets of glass includes transistors and pixel activation plates on its surface that serve to activate the liquid crystal material at discrete pixel locations. See U.S. Pat. No. 5,299,289 to Omae et al. for a description of conventional LCDs. A silicon-backed microdisplay is essentially a miniaturized LCD with a silicon die substituted for one of the glass sheets. The surface of the silicon die includes the transistors (typically CMOS-based), pixel activation plates, and drive circuitry (e.g. row and colunm drivers) required for operation of the silicon-backed microdisplay. The pixel activation plates also serve as reflector plates that reflect incoming light back towards a viewer during operation of the silicon-backed microdisplay.
A portion of a representative silicon-backed microdisplay is shown in cross-section in FIG. 1. Silicon-backed microdisplay portion 10 includes a silicon die 12, a silicon-side conductive layer 14 disposed on the silicon die 12, and a silicon-side passivation layer 16 disposed on the silicon-side conductive layer 14. Silicon-backed microdisplay portion 10 also includes a cover glass 18, and glass-side conductive layer 20 disposed on the bottom surface of the cover glass 18. As shown in FIG. 1, a uniform gap is present between the silicon-side passivation layer 16 and the glass-side conductive layer 20. This gap is filled with liquid crystal material 22. The combination of silicon die 12, silicon-side conductive layer 14, silicon-side passivation layer 16, cover glass 18, glass-side conductive layer 20 and liquid crystal material 22 essentially constitutes a miniature reflective mode, silicon-backed microdisplay.
To change the orientation of the liquid crystal material 22 and, therefore, affect the amount of light passing through the liquid crystal material 22, an AC electrical bias is applied between the silicon die 12 and the cover glass 18. Due to the electrical and surface properties of the cover glass 18, glass-side conductive layer 20, silicon-side passivation layer 16, silicon-side conductive layer 14 and silicon die 12, however, the microdisplay can exhibit flicker during operation. Such flicker, if visible to a user, renders the silicon-backed microdisplay unfit for commercial sale.
U.S. Pat. No. 5,764,324 to Lu et al. (hereinafter the '324 patent), which is hereby incorporated by reference, utilizes additional films coated on the silicon-side conductive layer to reduce flicker. The types of additional films which can be commercially applied on the silicon-side of a silicon-backed microdisplay are, however, limited by the nature of the silicon-side conductive layer, as well as by the need to provide predetermined transmissive and reflective optical properties on the silicon-side of the silicon-backed microdisplay. The approach described in the '324 patent, therefore, may not be capable of adequately reducing flicker. In addition, in the microdisplay described in the '324 patent, the glass-side conductive layer is uncoated, thereby allowing various impurities from subsequent processes to modify its surface properties (e.g., work function) in an uncontrolled manner.
Still needed in the field, therefore, is a silicon-backed microdisplay that provides for reduced flicker and also provides for the protection of its glass-side conductive layer during subsequent processing.